Easter data is mobile, since it falls on the Sunday following the first full moon after the spring equinox (the latter conventionally falls on March 21 for the Church, although the astronomical equinox fluctuates between 19 and 21).
The date of Passover is calculated from the beginning of the Jewish religious year (lunisolar calendar). The religious year began on the day of the first new moon after the spring equinox. Passover is celebrated 14 days after the start of the religious year, as specified in the Hebrew Bible.
The Christian Passover, theoretically, should be celebrated on the first Sunday after Passover, as specified in the Gospels. Instead, since 14 days after the new moon there is a full moon, the Council of Nicaea I established that Christian Easter must be celebrated on the first Sunday after the first full moon of spring.
This system was definitively established in the fourth century by the first council of Nicaea. In previous centuries there could have been different local uses on the date to follow, all however linked to the calculation of the Jewish Passover.
Easter and its mobile date: all the curiosities
In particular, some churches in Asia followed the tradition of celebrating Easter on the same day as the Jews, regardless of Sunday, and were therefore called Quartodecimans.
This gave rise to a dispute, called the Quartodeciman Easter, between the church of Rome and the Asian churches. Dionysius the Little calculated that the date of Easter is always between March 22nd and April 25th. In fact, if March 21st is a full moon day and falls on a Saturday, it will be Easter the next day (March 22nd).
If instead March 21st is Sunday, Easter day will be the following Sunday (March 28th). On the other hand, if the full moon falls on March 20th, the next will occur on April 18th, and if this day is a Sunday it will be necessary to wait for the following Sunday, that is, April 25th.
Easter is called low from March 22nd to April 2nd, medium from April 3rd to 13th and high from April 14th to 25th. In the Eastern Church, the calculation is made on the basis of the Julian calendar and therefore the Orthodox Easter can fall between April 4th and May 8th.
In the 1997 Ecumenical Council of Churches it was proposed to make the two dates coincide but the reform was not followed up. The tradition of the Catholic Church wants the date of Easter to be announced to the faithful by the celebrating priest during the rites of the feast of the Epiphany. Rapusia team wish all of you an happy Easter day!